My guess is that Keith David was disqualified from contention for The Expendables due to his obvious connections to Vin Diesel and the Riddick franchise. Schwarzenegger specifically chose to pass his torch to The Rock and snub The Deez, so it would have taken some intercession from Kurt Russell to get Arnold to sign off on letting David into the movie.
I think you're right, that David's voice would probably overqualify him for everything but the Blake role in Glengarry 2.0, which could only go to Arnold. That said, I would be in favor of a competing Glengarry remake project that had Keith David in the Blake role and David Keith in the Moss role. The two of them finally facing off would be like Heat times a million.